1. Field of the Invention
The present invention relates to a semiconductor device having a function to control a current to be supplied to a load by a transistor. In particular, the present invention relates to a display device including a pixel formed of a current driven light-emitting element in which luminance is changed in accordance with a current, and a signal line driver circuit therefor. Further, the present invention relates to a display device and an electronic device having the semiconductor device.
Note that the semiconductor device herein means all devices that can function by utilizing the semiconductor characteristics.
2. Description of the Related Art
In recent years, self-luminous display devices having pixels each formed with a light-emitting element such as a light-emitting diode (LED) have been drawing attention. As a light-emitting element used in such self-luminous display devices, there are an organic light-emitting diode (also referred to as an OLED (Organic Light-Emitting Diode), an organic EL element, an electroluminescence (EL) element, or the like), which have been drawing attention as a light-emitting element used for self-luminous display devices, and are used for an organic EL display or the like.
Since a light-emitting element such as an OLED is a self-luminous type, there are various advantages such that high visibility of pixels is ensured as compared to a liquid crystal display, no back light is required, higher response speed is achieved and the like. In addition, luminance of a light-emitting element is controlled by a current value to flow through there.
As for display devices with the use of such light-emitting elements which emit light by themselves (self-luminous type), there are known a simple matrix driving method and an active matrix driving method as a driving method thereof. The former method provides a simple structure, but has a problem that it is difficult to realize a large and high luminance display device or the like. Recently, an active matrix type has been developed actively, in which a current flowing to a light-emitting element is controlled by a thin film transistor (TFT) provided inside a pixel circuit.
Such an active matrix type display device has a problem that a current flowing to a light-emitting element is changed due to variation or degradation of current characteristics of a driving TFT, which leads to luminance variation.
In other words, the active matrix display device has a problem that a current supplied to a light emitting element varies due to variations in current characteristics of driving TFTs for a current flowing to a light-emitting element, thereby causing luminance variation. In view of this, various circuits are proposed, in which luminance variation is suppressed without changing a current supplied to a light-emitting element even when characteristics of driving TFTs vary between pixel circuits.
Non-patent document 1, patent document 1 and patent document 2 all disclose structures of semiconductor devices in active matrix type display devices. Patent documents 1 and 2 disclose circuit configurations in which current flowing to a light-emitting element is not changed due to variation of characteristics of a driving TFT disposed inside a pixel circuit. This configuration is called a current-writing type pixel or a current-inputting type pixel.
FIG. 92 shows an example of a basic configuration of a circuit diagram which has been disclosed in Patent Document 1 (International Publication No. WO 2004/061812) (refer to FIG. 14 of the patent document 1 and description in the specification regarding that). FIG. 92 shows a circuit including a current source transistor 9201, a shift transistor 9202, a switch 9203, a capacitor element 9204, a switch 9205, a switch 9206, a current source 9207, a load 9208, wires 9209, 9211, 9212 and a switch 9213.
FIG. 93 shows an example of a basic configuration of a circuit diagram which has been disclosed in Patent Document 2 (International Publication No. WO 2004/077671) (refer to FIG. 20 of the patent document 2 and description in the specification regarding that). FIG. 93 shows a circuit including a current source transistor 9301, a shift transistor 9302, a switch 9303, a capacitor element 9304, a switch 9305, switches 9306A, 9306B, current sources 9307A, 9307B, a load 9308, wires 9309, 9311A, 9311B, 9312 and a switch 9313.
FIG. 96 shows an example of a configuration of a circuit diagram disclosed in the non-patent document 1 (T. Shirasaki et al, SID '04 Digest, pp. 1516-1519 (2004)) (refer to FIGS. 2 and 3 of the non-patent document 1 and description regarding that). A circuit diagram shown in FIG. 96 is a diagram in which the circuit diagram shown in the non-patent document 1 is described along with description of patent documents 1 and 2. FIG. 96 shows a circuit including a current source transistor 9601, a switch 9603, a capacitor element 9604, switches 9605, 9606, a current source 9607, a load 9608, and wires 9609, 9611 and 9612.
In the case of circuit configurations shown in Patent Documents 1 and 2, a switch is required between the current source transistor and the load, and thus, there is a problem that a current is difficult to flow into the load. When the switch is not provided, a current flows into the load, and thus, a desired operation cannot be obtained. Further, in the case of the circuit configurations shown in Patent Documents 1 and 2, there is a problem that the number of transistors is large, which leads to decrease of aperture ratio.
A problem of the circuit configuration shown in FIG. 92 is described with reference to FIGS. 94A and 94B. FIG. 94A shows the same circuit configuration as FIG. 92. In FIG. 94A, the switch 9203 is denoted by SW1; the switch 9205, SW2; the switch 9206, SW3; a potential of the wire 9209, Vdd; a potential of the wire 9211, Vss1; a potential of the wire 9212, Vss2; and the switch 9213, SW4 for explanation. The potentials of the wires are set Vdd>Vss2>Vss1.
FIG. 94B shows a change of a potential of each switch and each wire, at a signal writing operation and an input operation in a circuit configuration of FIG. 94A.
At the signal writing operation of FIG. 94B, SW1 turns ON, SW 2 turns ON, SW3 turns ON and SW4 turns OFF. Current supply to the power source 9207 starts and a gate-source voltage (Vgs) which is necessary for the power source transistor 9301 to supply current to the power source 9207, is applied to the capacitor element 9204. At that time, a nodeG has a potential of Vdd, and thus, a potential of a nodeS is increased, since the capacitor element 9204 holds a voltage of Vgs. The potential of the nodeS comes to be in a steady state, and the difference between the nodeG and the nodeS becomes Vgs. In FIG. 94B, Vload is applied to a load 9208 between the nodeS and Vss2 at the signal writing operation, and a potential gradient from the nodeS to Vss is generated. Thus, a current flows to the load 9208 at a signal writing operation, thereby causing such a defect that a right signal cannot be supplied to the load.
In the case of a circuit diagram described in the non-patent document 1, a potential of a power supply line for supplying a current to a display element which is a load, is changed one row by one row, thereby preventing current from flowing into the display element at the signal writing operation to the load. This is because a right signal cannot be input into a pixel, when the current is supplied to the display element which is a load at the signal writing operation. As a result, defective display is caused. In addition, when the frequency is F, the capacitance is C and the voltage is V, power consumption P is obtained by the formula (1), generally.P=FCV2(F: frequency,C: capacitance,V: voltage)  (1)
Thus, according to the formula (1), the frequency is preferably small so as to reduce power consumption. In other words, it is not preferable that the potential of the power supply line is changed, which results in increase of power consumption.
In the non-patent document 1, it is necessary that a large current is supplied to the light-emitting element from the power supply line. Thus, it is necessary to dispose a switch which can control a large current, so as to change a potential of the power supply line of each row and to supply a large current. Thus, there is a problem that a size of a transistor for a circuit is needed to be large. If the size of a transistor is large, power consumption of a transistor also becomes large. In a conventional configuration as described in the non-patent document 1, in a transistor for driving a display element at the signal writing operation, Vds=Vgs is obtained, while Vds>Vgs is obtained at the time of light emission. When constant current characteristics (flatness of current) of a transistor in a saturation region becomes worse, current values at the signal writing operation and at the time of light emission become extremely different.
In the invention of the non-patent document 1, currents flowing into a transistor are equal at the signal writing operation and at the time of light emission. Therefore, for example, when a dark image is to be displayed, in the case where a slight amount of current is written into a transistor, there is a problem that a signal is not written sufficiently, because of an influence of noise, parasitic intersection capacitance or wire resistance.